At his campaign office here the other day, Mr. Lugar smiled politely while volunteers boxed up thousands of yard signs to distribute to Indiana’s 92 counties. He spoke with admiration of the sophistication of his campaign’s mobile, computerized, microtargeting phone bank, with which, he explained, volunteers “can just press a button and they’re on line with somebody.” He described how his campaign had, in essence, gone “to school” on the methods of campaigns around the country.

Mr. Lugar, who has not had a primary challenger since he first won election in 1976 and last contended with a race where the margin was close in 1982, is locked in a Republican primary fight for the seat he has held for six terms with the May 8 election fast approaching. A poll conducted late last month, the Howey/DePauw Indiana Battleground Poll, showed Mr. Lugar leading Richard E. Mourdock, the state’s treasurer, 42 percent to 35 percent among likely primary voters, an advantage that is within the poll’s margin of sampling error of plus or minus five points.

Craig Dunn, the Republican chairman in Howard County, said he was stunned last year when he asked the 15 members of his local steering committee how many would vote for Mr. Lugar. “Not a hand went up,” said Mr. Dunn, who has supported Mr. Mourdock, as did, his campaign said, nearly three-quarters of the party’s county chairmen back when he announced plans to run more than a year ago. (Mr. Lugar’s supporters say those numbers have since shifted and shrunk.) “This never would have happened to Dick Lugar in his prime,” Mr. Dunn said.

Mr. Lugar sees his troubles as a product of forces outside Indiana. “You can say, ‘Why in the world are we having such a time?’ ” he said. Then he offered an answer to the question: “Because there are others in America who are very interested in this, sort of as a battleground, or I’d even say a playground, for their thoughts.”

To hear others tell it, Mr. Lugar, the product of a more genial era of politics, faces a confluence of opposition. Tea Party groups and organizations like the Club for Growth and the National Rifle Association are questioning his conservative credentials, some pointing to positions he has taken in favor of the bank bailout, President Obama’s Supreme Court nominees, the New Start nuclear arms control treaty, and more.

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Senator Lugar has rarely had to debate, but last week he prepared to face State Treasurer Richard E. Mourdock.Credit
Peter W. Stevenson for The New York Times

Meanwhile, some Indiana residents, including active members of the Republican Party, say they wonder whether Mr. Lugar, in all those years in Washington and around the world in his influential role on the Foreign Relations Committee, has failed to come home enough for ordinary Lincoln Day dinners and the like. Along the way, some say, he lost touch. And just below the surface is an uncomfortable question about age and how long in Washington is too long.

Beyond Indiana, much is at stake. Democrats hoping to hold on to a majority in the Senate see a glint of opportunity to take a Republican seat — a possibility that polls suggest is more likely if Mr. Lugar loses and leaves Mr. Mourdock, 60, to face Representative Joe Donnelly, a Democrat, in November. Tea Party members view the potential defeat of Mr. Lugar as a crucial chance to prove their muscle as some observers try to declare their movement over.

“If we win, the Tea Party goes into a higher level as a credible force,” said Greg Fettig, a supporter in Indiana. Last fall, he said, 55 of the state’s Tea Party groups convened and overwhelmingly favored Mr. Mourdock. Organizers at FreedomWorks, a national group that has helped build the Tea Party movement and has assisted efforts here, have already begun likening this race to Mike Lee’s defeat of Senator Robert F. Bennett, a fellow Republican, in Utah in 2010.

But others say that the strength and unity of the Tea Party here have been overblown, and that Mr. Lugar, a former Eagle Scout, Rhodes scholar and Navy officer, has clear wells of strength. These include more money in the bank than his opponent and support from Mitch Daniels, the popular governor who, as a student, worked for Mr. Lugar when he was mayor of Indianapolis in the 1960s and ’70s and stayed on for years.

Mr. Daniels, who asked Mr. Lugar to be godfather to one of his daughters, fended off assertions that Mr. Lugar might not be conservative enough or had compromised too often across party lines. Mr. Lugar’s supporters say he has pressed for less government spending and a balanced budget amendment, and fought President Obama’s health care overhaul and regulations that stifle business.

“It’s ironic and it’s just inaccurate to suggest that somehow he’s not very strongly Republican in his viewpoints,” Mr. Daniels said.

Lately, Mr. Lugar has taken blow after blow. A challenge that he did not meet a residency requirement for candidates because he lives much of the time in McLean, Va., failed. However, he was required to change his voter registration to the farm his family has owned for decades, rather than the Indianapolis house that he sold in 1977. Then his office announced that he was returning $14,500 to the government for nights he had spent in Indiana hotel rooms during adjournments — a technical oversight of a Senate expenses rule, but one more reminder of his long time away.

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Senator Richard G. Lugar’s headquarters in Indianapolis.Credit
Peter W. Stevenson for The New York Times

All the while, the onslaught of ads and critiques came, denouncing Mr. Lugar as a friend of President Obama, recipient of an F-rating from the N.R.A., and someone who once opposed a ban on earmarks and supported the Dream Act.

Mr. Lugar, who dismissed claims of a closeness to President Obama, is unapologetic for working the other side of the aisle, an approach that in the 1990s brought the accomplishment for which he may be best known — a program, with Sam Nunn, a Democratic senator, for disarmament in the former Soviet Union.

“It’s a fact of life,” Mr. Lugar said, “if you are a legislator for any period of time, and if you are attempting to pass what you believe is very constructive legislation for the country, either domestically or in terms of foreign policy, that in the Congress of the United States, you’re going to deal with members of the other party.”

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Mr. Mourdock has told audiences, like the one at a recent Rotary breakfast in Noblesville, that bipartisanship has taken the nation to the brink of bankruptcy. “The time for being collegial is past,” Mr. Mourdock said in an interview. “It’s time for confrontation.”

A former coal executive who remains far less known among Indiana voters, Mr. Mourdock lauded Tea Party members for their support and grew teary-eyed when reflecting on the devotion of a large Tea Party group he addressed several years ago. Still, Mr. Mourdock, who first won election as state treasurer in 2006, recoils at the way he says Mr. Lugar has tried to paint him: in Mr. Mourdock’s words, as a “wild-eyed Tea Party candidate.”

Back inside Mr. Lugar’s campaign office last week, volunteers gushed over his debate performance a night earlier, his first such debate in a dozen years after Democrats did not even field an opponent in 2006. Pamela Altmeyer Alvey, a volunteer, recalled how friends, including one who is upward of 80, had voiced doubt about Mr. Lugar before the debate but sounded different now. “They said, ‘He was so vibrant!’ ” Ms. Altmeyer Alvey told the senator.

If finding himself in a battle now, in his 36th year in the Senate, feels insulting or painful or a little awkward, Mr. Lugar is not saying. “I’ve long since forgotten about whether it’s odd,” he said. “This is just what I do all my life. And so we just take each day as happily as possible, look at it as optimistically as we can.”

A version of this article appears in print on April 17, 2012, on Page A1 of the New York edition with the headline: Once Every 36 Years, a Primary Fight for Lugar. Order Reprints|Today's Paper|Subscribe